Carbon nanotubes (CNTs) have recently attracted attention because of their unique physical properties. CNTs are light-weight and have high mechanical strength, good thermal conductivity, large surface area, and a high aspect ratios. CNTs with variable electronic properties can be obtained by introducing chirality (the rotation of the symmetry of carbon network along the cylinder axis) in individual tubules, whereby the electronic properties may be varied as a function of their chirality. CNTs have superior electron emitting properties compared to conventional materials. Typical field enhancement factors ranging between 30,000 and 50,000 can be obtained for individual CNT tubules, and between 1,000 and 3,000 for CNT arrays. The field enhancement factors of CNT arrays are much smaller than those for individual CNT tubules to the presence of a planar substrate that is necessarily required for physically supporting the CNT tubules forming an array. The field enhancement factors of dense CNT arrays (large number of CNT tubules per unit area substrate) are even smaller because the electrical field around one tubule is screened by neighboring tubules due to their close proximity.
Utilizing CNTs for microelectrode arrays (MEAs) has not been accomplished in the art. MEAs consisting of hundreds of metal microelectrodes (MEs) with diameter of several micrometers (μm) have been fabricated by lithographic techniques. MEAs show many advantages over the conventional macro electrodes. For example, MEAs have high mass sensitivity, increased mass transport and the decreased influence of solution resistance. However, present MEAs are limited by their poor detection limits and low signal-to-noise (S/N) ratio. Noise level depends on the active surface area of the individual electrode whereas the signal depends on the total surface area of all electrodes. Present MEAs have an inadequate number of electrodes per unit area for offering acceptable S/N ratios. Increasing the number of electrodes per unit area will therefore result in an increase in the S/N ratios and improve detection limits. Arrays of vertically aligned CNTs have the good material properties and size (about 20 nm to about 200 nm) for MEAs, but do not have the needed inter-tubule distances within the array (site density). The spacing between individual CNT tubules needs to be sufficiently large in comparison with the diameter of the individual CNT tubules to make each CNT tubule work as an individual nanoelectrode. Reducing the size of each individual electrode to nanometer scale and increasing the total number of electrodes per unit area should improve detection limits and S/N ratio.
For aligned CNTs, tuning of CNT characteristics such as diameter, length and inter-tubule distances within the array (site density) is important for certain electrical and electronic applications, such as field emission and nanoelectrode arrays, due to the shielding effect in a dense array. Since the field enhancement of carbon nanotube film is affected by the length of carbon nanotubes and the spacing between them, it is important to characterize the effect of length and spacing on field emission properties in order to obtain a high and uniform field emission current at low electric field. The effect of length and spacing is known for randomly oriented CNT films and vertically aligned carbon nanotube films. The utility of such randomly oriented CNT films is limited because CNT tubule length, CNT tubule diameter, and CNT tubule spacing cannot be independently varied, so that effect of CNT tubule length, CNT tubule diameter, and CNT tubule spacing (site density) on field enhancement, were not clearly independent. The independent control of such parameters which is important for field enhancement, is therefore not possible using the methods known in the art.
Syntheses of CNTs wherein the individual tubules are organized in a random or aligned manner are known in the art. Although tubule diameter and tubule length of aligned CNTs have been controlled by controlling site densities, catalyst particle size and the growth time, respectively, such methods have been unsuccessful in controlling CNT densities. Furthermore, the present methods which attempt to reduce catalytic site density by electron beam lithography, photolithography, micro contact printing, and shadow mask techniques have limited utility especially in terms of commercial viability since they require expensive equipment, are labor intensive, do not achieve uniform site density control over large areas, and are not amenable to incorporation into large-scale production processes.
There is therefore a need for obtaining aligned CNT arrays with controllable site densities over relatively larger surface areas.